EDIT: As of 22/04/2014, there's now a manual version of the mod for Minecraft 1.7.9. It's available at the Curse page, linked below, and there's a version both for the standalone server and for the regular game client. If you don't know how to do manual mods, consult the modding FAQ
I was always a fan of good old boosters, and was rather sad when Notch removed them, and sadder still when powered rail got added and wasn't nearly as good.
I recently got back into Minecraft, and decided that this was a perfect target for modding.
It's possible I've stuffed something up packaging this, but it works on my system. Requires Forge 1.7.10 for the 1.7.10 version, the 1.7.9 version is a manual mod and doesn't need any modloader. Only the server needs the mod. The forge mod works with Forge 1.7.2 as well.
Added a simple picture that's just a standard booster setup. Might make a video at some point - pictures probably aren't the best mechanism for describing the behaviour change.
Redcat: In the initial post I linked to the Minecraft Wiki description of boosters. Here it is again. The tl;dr of it is that in days long past, before powered rail was added to the game, minecrafters discovered that certain configurations of rail could cause two minecarts in blocks next to each other to collide with each other, pushing both minecarts forward - where they're still colliding, and so they push each other forward, and so on, 'boosting' minecarts up to the maximum minecart speed very quickly, and then building up momentum linearly with additional contact time. The bug ended up being exploited widely to construct railway systems of significant complexity, before it was removed. Powered rail was implemented to solve the same problem as boosters, but personally I find it isn't as interesting, for several reasons - it requires less thought when designing railway systems, there's less scope for design variation for different use-cases, it's much more expensive, and also it's significantly less powerful than boosters.
The wiki describes the exact configuration required to boost carts, but a simple check for mod presence is this: Do minecarts on adjacent rails collide with each other at all? If they do, the mod is working.
Strange, I just double-checked to make sure I wasn't crazy, and both the corner-turning and downslope boosters work fine in my singleplayer creative test world, singleplayer survival test world, and multiplayer connecting to a server with the mod, where the client doesn't have the mod. If you're using it on a server the server needs the mod, not the client.
When you start minecraft, is boostermod listed under the 'mods' menu from the main screen? Also, are you running any other mods?
There's now a manual 1.7.9 version of the mod, if you don't mind copying .class files into jars and generally making a huge mess.
Note for anyone trying to do this yourself, because I've been hit by a truck or don't have a version for the version of minecraft you want to play or whatever: The only class that needs to be touched is the minecart class (xk.class in 1.7.9). The bit of code you're looking for is the collision handling for minecart-minecart collisions. It looks something like this in 1.7.9:
It's calculating the angle between the line between the two minecarts and the minecart's travel direction using the dot product, and if that angle is more than arccos(0.8) (about 30 degrees IIRC), they don't collide. Commenting out that code allows for boosters. " < 0.800000011920929D" is probably the best search string to use.
My friend, I love you. I tried the 1.7.2 version of the mod with a very basic booster and it worked like a charm!
Gonna try out the more complicated stuff and see if it all still works - I was so sad when this "bug" was "fixed"! Powered rail is boring